Creating points in specified distance along line using QGIS

Question

I want to use QGIS creating points along the line, but in specified distance. I found two GRASS tools in SEXTANTE Toolbox：

• v.to.points
• v.segment

I tried v.to.points, setting the maximum distance between points in map units to 100 and got this：

The second photo showed a close look from the upper one. The result was what I want, but I want to get less points, but even I set the maximum distance to 1000, or 1000000, the result was the same.

I tried to use v.segment, but I had no file containing segment rules.

Underdark gave me some advice, and I tried Densify geometries given an interval. First it gave me a polyline shapefile the same as I input, and then I used extract nodes to try to get those points. But the result was the same as I got using v.to.points, no matter what I set the distance interval.

How can I create points along the line, and in specified distance？

Answer 1

There is now a plugin called QChainage that does exactly what you want. You specify an interval distance and the plugin creates points along your line at the specified interval.

Answer 2

In the Python console:

• you can use the Shapely module (as in How to create equidistant points in QGIS?) with the function

  point = line.interpolate(currentdistance)

• The new Python API of the master version (1.9) has an equivalent command (as in Generating chainage (distance) nodes in QGIS of Nathan Woodrow)

  point = geom.interpolate(currentdistance)

• or you can use vector algebra and direction cosines (as in PyQGIS: geometries, vectors, vector algebra or direction cosines, application examples, in French)

1) create a memory layer class (complete class (without attributes) at crea_mem_layer.py)

class Create_vlayer(object):
    '''creation of a virtual layer''' 
     def __init__(self,nom,type):
         self.type=type
         self.name = nom
         self.layer =  QgsVectorLayer(self.type, self.name , "memory")
         self.pr =self.layer.dataProvider() 
     def create_point(self,geometry):
         # add point to the layer
         self.seg = QgsFeature()
         self.seg.setGeometry(QgsGeometry.fromPoint(geometry))
         self.pr.addFeatures([self.seg])
         self.layer.updateExtents()
     @property
     def display_layer(self):
         #end of layer and display layer 
         QgsMapLayerRegistry.instance().addMapLayers([self.couche])

2) vector algebra functions (from algèbre_vect_PyQGIS.py)

import math 
def mag(point):
    # magnitude of a vector
    return math.sqrt(point.x()**2 + point.y()**2)
def diff(point2, point1):
    # substraction betwen two vector
    return QgsPoint(point2.x()-point1.x(), point2.y() - point1.y())
def length(point1,point2):
    # with PyQGIS: sqrDist
    return math.sqrt(point1.sqrDist(point2))

3) direction cosines

def dircos(point):
    cosa = point.x() / mag(point)
    cosb = point.y()/ mag(point)
    return cosa,cosb

4) process line or line segments

def pairs(list):
    # list pairs iteration 
    for i in range(1, len(list)):
    yield list[i-1], list[i]

layer = qgis.utils.iface.activeLayer()
# interval between points
interval = 5 m
# create virtual layer
gen_pt  = Create_vlayer("mid5", "Point")

for elem in layer():
    line = elem.geometry()
    for seg_start, seg_end in pairs(line.asPolyline()):
       line_start = QgsPoint(seg_start)
       line_end = QgsPoint(seg_end)
       # mid point = vector coordinates [x2-x1,y2-y1]
       pointm =diff(line_end, line_start)
       # direction cosines of the segment
       cosa,cosb = dircos(pointm)
       # length of the segment
       lg = length(line_end, line_start)
       # generate and add points to the virtual layer 
       for i in range(interval,lg,interval):
           gen_pt.create_point(QgsPoint(line_start.x()  + (i * cosa), line_start.y() + (i*cosb)))

# display layer
gen_pt.display_layer

Results

polyline, equidistant point with Shapely or PyQGIS2, with direction cosines

Then just adjust the interval

Answer 3

In QGIS 3 (at least in QGIS 3.10) there is a processing tool for that purpose: "Points along geometry"

Answer 4

I've written a script that changes the Sextante Densify geometries tool to accept a certain distance. It's called Densify geometries given an interval.

After running Densify, you can extract the points using Extract nodes tool.

You can get it from Github and install instructions are on my blog.

Answer 5

If you don't want to horse around with python scripts you can simply install and use the "Profile from line" plugin and ignore / delete the raster value column. The chainage will be whatever you set teh sampling interval to be.

Answer 6

Well not sure if this is in the stable release, but in the sextante toolbox on 1.9 alpha under Geoalgorithims->Vector is the "Convert Lines to Points" option. Works a treat, be great if it added a field for distance along the line as well though.

Interesting it did add the NAME field from my vector layer though.

I tried getting that Python Script by Nathan Woodrow working, but I suck with python. And with code in general it seems.

Answer 7

I spent a long time struggling with this issue, but eventually got more-or-less what I was looking for using QChainage. What I would like to share is something that may help others. When using the SAGA geoprocessing dissolve tool on the line, QChainage produces incorrect results. Use of a different dissolve tool causes it to work fine.